34 THE AMERICAN NATURALIST [Vol. XLIV 



known formation at Elko Station, Nevada, I do not find 

 anything which really seems to correspond with Floris- 

 sant. According to the theory outlined above the Mascall 

 beds of Oregon, which possess a varied flora, should be 

 either contemporaneous or (more probably) somewhat 

 earlier. Fortunately, fourteen species of mammals have 

 been obtained from the Mascall, and these place it rather 

 definitely in the Middle Miocene. Considering, therefore, 

 a probable moderate difference in time, combined with 

 noteworthy geographical and altitudinal differences, we 

 ought to find the Mascall flora similar to, but by no means 

 identical with, that of Florissant; and this is exactly what 

 comparisons show. 



Thus of the 77 Mascall plants (nearly all trees) re- 

 ferred to definite genera, no less than 56 are congeneric 

 with those of Florissant. Of those not congeneric, five 

 are so dubious that they have not been specifically deter- 

 mined. The Mascall genera not yet found at Florissant 

 are the following: 



1. Equisetum.-This has no significance, as it abounds 

 in Colorado to-day, and must have been present during 

 the Florissant period. 



